1. Field of the Invention
The present invention relates to the field of chemical mechanical polishing. More particularly, the present invention relates to methods and apparatus for chemically mechanically polishing substrates, such as semiconductor substrates, on a rotating polishing pad in the presence of a chemically active and/or physically abrasive slurry, and providing a fresh supply of slurry onto the face of the substrate engaged on the polishing pad as the substrate is being polished. Additionally, the invention may include a pad conditioning apparatus to condition the polishing pad while the pad is being used to polish substrates.
2. Background of the Art
Chemical mechanical polishing is a method of polishing materials, such as semiconductor substrates, to a high degree of planarity and uniformity. The process is used to planarize semiconductor slices prior to the fabrication of microelectronic circuitry thereon, and is also used to remove high elevation features created during the fabrication of the microelectronic circuitry on the substrate. One typical chemical mechanical polishing process uses a large polishing pad, located on a rotating platen, against which a substrate is positioned for polishing, and a positioning member which biases and positions the substrate on the rotating polishing pad. A chemical slurry, which may also include abrasive materials therein, is maintained on the polishing pad to modify the polishing characteristics of the polishing pad to enhance the polishing of the substrate.
The use of chemical mechanical polishing to planarize semiconductor substrates has not met with universal acceptance, particularly where the process is used to remove high elevation features created during the fabrication of microelectronic circuitry on the substrate. One primary problem which has limited the use of chemical mechanical polishing in the semiconductor industry is the limited ability to predict, much less control, the rate and uniformity at which the process will remove material from the substrate. As a result, chemical mechanical polishing is a labor intensive process, because the thickness and uniformity of the substrate must be constantly monitored to prevent over-polishing or inconsistent polishing of the substrate surface.
One factor which contributes to the unpredictability and non-uniformity of the polishing rate of the chemical mechanical polishing process is the non-homogeneous replenishment of slurry at the interface of the substrate and the polishing pad. The slurry is primarily used to enhance the material removal rate of selected materials from the substrate surface. As a fixed volume of slurry in contact with the substrate reacts with the selected materials on the substrate surface, the fixed volume of slurry becomes less reactive and the polishing enhancing characteristics of that fixed volume of slurry are significantly reduced. One approach to overcoming this problem is to continuously provide fresh slurry onto the polishing pad. This approach presents at least two difficulties. Because of the physical configuration of the polishing apparatus, introducing fresh slurry into the area of contact between the substrate and polishing pad is difficult, and providing a consistently fresh supply of slurry to all portions of the substrate is even more difficult. As a result, the uniformity and overall rate of polishing are significantly affected as the slurry reacts with the substrate.
Several methods have been proposed for maintaining fresh slurry at the substrate-polishing pad interface. One method allows the substrate to "float" on the polishing pad. The object of floating the substrate on the polishing pad is to provide a very small downwardly directed force at the substrate-polishing pad interface, so that slurry will flow between the substrate and the polishing pad. This method is ineffective because the slurry is still substantially prevented from moving under the substrate by surface tension and other factors, and the use of a low force at the substrate-polishing pad interface substantially increases the cycle time necessary to polish a substrate.
Another method of providing slurry to the face of the substrate engaged against the polishing pad uses a plurality of holes in the platen, and the slurry is injected through the holes and underside of the polishing pad. The object of this method is to ensure that the slurry is constantly replenished at the substrate-polishing pad interface through the underside of the polishing pad. Although this method does provide slurry to the face of the substrate engaged against the polishing pad, it has several drawbacks. The primary problem encountered when using this method is that the slurry is injected over the entire area of the polishing pad. Therefore, substantial areas of slurry wetted polishing pad are exposed to the ambient environment, and the slurry that is exposed to the environment tends to dry and glaze the surface of the polishing pad. This glazing significantly reduces the ability of the pad to polish the substrate, and therefore reduces the effectiveness of the polishing equipment.
A further method of providing slurry to the substrate-polishing pad is shown in U.S. Pat. No. 5,216,843. In this reference, a plurality of concentric, circular grooves, which have a center that is co-terminus with the axis of rotation of the polishing pad, are provided in the upper surface of the polishing pad. Additionally, radial "microgrooves" are continuously formed in the surface of the polishing pad by a pad conditioning apparatus. The microgrooves serve to condition the polishing pad surface. Both the polishing pad and the substrate rotate as the substrate is processed. Because the substrate rotates, all areas on the surface of the substrate will pass over one, or more, of the grooves during each substrate rotation. However, despite the fact that all areas of the substrate will pass over one or more grooves, the slurry is still non-uniformly replenished on the substrate. In particular, where the substrate is rotated on the rotating polishing pad, zones of high and low slurry replenishment will occur on the face of the substrate because different areas on the substrate will pass over different numbers of grooves as the substrate rotates. If the substrate is not rotated, but is instead reciprocated in a linear or arcuate path, the relative distribution of fresh slurry will vary as the distance on the substrate from a groove increases from the nominal position of the substrate on the polishing pad. Therefore, the frequency at which fresh slurry reaches each location on the substrate varies across the face of the substrate, which leads to zones of high and low material removal on the substrate. In particular, where the substrate is linearly or arcuately reciprocated over a distance less than one-half of the spacing between the concentric grooves, portions of the substrate will not come into contact with any groove area, and thus discrete areas of very low slurry replenishment will occur on the substrate.
In addition to the affect of slurry distribution on the rate and uniformity of polishing, the polishing characteristics of the polishing pad also are affected by glazing and compression of the polishing pad surface. This glazing and compression are natural by-products of the polishing process and typically cause open cells on the polishing pad surface to close by (i) compression or (ii) filling with polished substrate particulates and dried slurry. Once the polishing rate of the particular pad-slurry combination is sufficiently affected by these factors, the polishing pad is either replaced or conditioned with a conditioning wheel, conditioning arm, or other apparatus. During this conditioning step, the substrate is removed from the polishing pad, so no polishing occurs. This reduces the throughput of substrates through the chemical mechanical polishing apparatus, leading to higher processing costs.
One method of conditioning the polishing pad while simultaneously polishing substrates is shown in U.S. Pat. No. 5,216,843. In that reference, a "stylus" type of conditioner is provided to constantly cut "microgrooves" in the polishing pad surface. The stylus sweeps radially inwardly and outwardly as the polishing pad rotates under the stylus head, and thus a zig-zag path of freshly opened cells is cut into the polishing pad. This system has several disadvantages. First, the stylus is delicate and subject to breakdown. Second, the cutting action of the stylus is difficult to control. Finally, the path cut by the stylus is very small and is therefore of limited practical utility in conditioning the polishing pad.
Thus, there exists a need to provide a chemical mechanical polishing apparatus with better slurry distribution and improved pad conditioning.